1. Field of the Invention
The present invention relates to a receiving method and a receiving apparatus for receiving a signal subjected to an orthogonal frequency division multiplexing (to be referred to as an OFDM hereinafter) modulation and, more particularly, to a technique for detecting a frequency error of a received signal.
2. Description of the Related Art
As one of modulation systems used when digital data having a relatively large capacity is transmitted in a wireless mode or the like, OFDM modulation is practically used. The OFDM modulation system is a system for transmitting transmission data as a multi-carrier which is dispersed into a plurality of sub-carriers. This modulation system can efficiently transmit a large-capacity data in a wireless mode.
As a wireless transmission state of a signal subjected to the OFDM modulation, there may be such a case that one unit of OFDM modulation signal is continuously transmitted in a predetermined frequency band. For example, as shown in FIG. 1, in a wireless telephone system, it is assumed that OFDM modulation signals transmitted in a wireless mode from a plurality of mobile stations MS1, MS2, MS3, and MS4 are subjected to a receiving process by a base station. In this case, the OFDM modulation signals from the respective mobile stations MS1 to MA4 are transmitted as multi-carrier signals respectively using different bands F1 to F4, and all the bands F1 to F4 are subjected to a receiving process by the base station. A system in which multi-connection between the base station and a plurality of surrounding mobile stations is made by using OFDM modulation waves as described above has been proposed.
By the way, as shown in FIG. 1, when signals transmitted from a plurality of mobile stations are subjected to a receiving process by a base station, a frequency offset of respective transmission signals must be detected. FIG. 2 is a diagram showing an example of a circuit arrangement for detecting a frequency offset in a conventional base station. This example is a case wherein the frequency offsets of signals from the four mobile stations MS1 to MS4 shown in FIG. 1 are detected. A receiving processor 2 connected to an antenna 1 performs the signals of the transmission bands at once, and then the received signals are supplied to four band-pass filters 3a, 3b, 3c, and 3d having different passing bands, respectively. In this case, the passing bands of the band-pass filters 3a to 3d are set for the transmission bands F1 to F4 shown in FIG. 1, respectively.
Outputs from the band-pass filters 3a to 3d are supplied to different frequency offset detection circuits 4a, 4b, 4c, and 4d, respectively. The frequency offsets of transmission signals from the four mobile stations MS1 to MS4 are independently subjected to a detection process by the frequency offset detection circuits 4a to 4d, respectively. On the basis of amounts of frequency offsets detected by the frequency offset detection circuits 4a to 4d, the following correction process is performed. That is, the reception frequencies of signals transmitted from the mobile stations are corrected, or data for correcting a frequency offset is transmitted to a corresponding mobile station.
As a frequency offset detection process performed in each of the frequency offset detection circuits 4a to 4d, for example, a detection process using a cycle prefix component included in an OFDM modulation signal is known. An example of a frequency offset detection arrangement using the cycle prefix component is shown in FIG. 3. A signal from a terminal 5 at which an output from each of the band-pass filters 3a to 3d can be obtained is supplied to a multiplier 7 through a delay circuit 6 for delaying the signal for a predetermined period, and a signal obtained at the terminal 5 is directly supplied to the terminal 5. Both the signals are subjected to a multiplying process. In this case, an amount of delay of the delay circuit 6 is set to be an inherent amount of delay included in the cycle prefix component. The relationship between the cycle prefix component and the amount of delay will be described later. The signal delayed by the delay circuit 6 is a complex signal. A multiplying process of a complex conjugate is performed in the multiplier 7.
An output from the multiplier 7 is supplied to an averaging circuit 8 to calculate an average in a predetermined period, and the average value is supplied to an output terminal 9 as a value which is in proportion to a frequency offset value. The period in which averaging is performed by the averaging circuit 8 is set to be a period corresponding to one length (time) of a cycle prefix component, for example. With such a process, a value which is in proportion to the frequency offset value is obtained at the output terminal 9.
A cycle prefix component included in an OFDM modulation signal will be described below. First, a complex sine wave is defined by a function rot() shown by Equation (1) equation.
rot(s)=exp(j2xcfx80s)xe2x80x83xe2x80x83(1)
At this time, an OFDM modulation signal before windowing for transmission can be described as represented by the following Equation (2) equation.                               x          ⁡                      (            t            )                          =                              ∑                          k              =              a                        b                    ⁢                                    x              k                        ⁢            rot            ⁢                          xe2x80x83                        ⁢                          (                              k                ⁢                                  xe2x80x83                                ⁢                                  f                  c                                ⁢                t                            )                                                          (        2        )            
Heare, symbol xk denotes a transmission symbol (transmission symbol put on a kth sub-carrier), and symbol fc denotes a sub-carrier interval. The OFDM modulation signal represented by Equation (2) and obtained before windowing is performed can also expressed as represented by the following Equation (3).                                                                         x                ⁡                                  (                                      t                    +                                          T                      c                                                        )                                            =                              xe2x80x83                            ⁢                                                ∑                                      k                    =                    a                                    b                                ⁢                                                      x                    k                                    ⁢                  rot                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          k                      ⁢                                              xe2x80x83                                            ⁢                                                                        f                          c                                                ⁡                                                  (                                                      t                            +                                                          T                              c                                                                                )                                                                                      )                                                                                                                          =                              xe2x80x83                            ⁢                                                ∑                                      k                    =                    a                                    b                                ⁢                                                      x                    k                                    ⁢                  rot                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  k                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          c                                                ⁢                        t                                            +                                                                        f                          c                                                ⁢                                                  T                          c                                                                                      )                                                                                                                          =                              xe2x80x83                            ⁢                                                                    ∑                                          k                      =                      a                                        b                                    ⁢                                                            x                      k                                        ⁢                    rot                    ⁢                                          xe2x80x83                                        ⁢                                          (                                              k                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          c                                                ⁢                        t                                            )                                                                      =                                  x                  ⁡                                      (                    t                    )                                                                                                          (        3        )            
In this case, Tc=1/fc is established, and the OFDM modulation signal is represented by a periodic function of a period Tc depending on the sub-carrier interval fc. More specifically, for example, when fc=4.1666 [kHz]=100 [kHz]/24, Tc=240 [xcexcs]. In this case, an OFDM signal obtained before windowing is performed has, for example, as shown in FIG. 4, a signal waveform having a periodicity of 240 [xcexcs].
When the OFDM modulation signal is to be transmitted, the process for multiplying windowing data (time waveform) called a transmission window. A signal obtained by multiplying the windowing data is represented by the following equation. In this equation, w(t) denotes windowing data (window).                               y          ⁡                      (            t            )                          =                              ∑                          k              =              a                        b                    ⁢                                    x              k                        ⁢            rot            ⁢                          xe2x80x83                        ⁢                          (                              k                ⁢                                  xe2x80x83                                ⁢                                  f                  c                                ⁢                t                            )                        ⁢                          w              ⁡                              (                t                )                                                                        (        4        )            
FIG. 5 is a diagram showing an example of windowing data. As described above, when Tc=240 [xcexcs], windowing data based on the time is generated. In this case, for simplifying the description, a window is represented by an rectangular wave, and the window is set to be a window in a period L (280 [xcexcm]) obtained by adding a period Tg (40 [xcexcs]) to the above-mentioned period Tc (240 [xcexcs]). As an OFDM modulation wave multiplied by the window, an OFDM wave in the period Tg at the start portion of one unit of window and an OFDM wave in the period Tg, at the end portion, having a time width equal to that of the period Tg at the start portion have completely an equal waveform. This modulation wave is called a cycle prefix. This cycle prefix corresponds to a cycle prefix CP shown in FIG. 5.
The OFDM wave multiplied by the windowing data is represented by the following equation when a frequency offset of v [Hz] is put on the OFDM wave.                               y          ⁡                      (            t            )                          =                              ∑                          k              =              a                        b                    ⁢                                    x              k                        ⁢            rot            ⁢                          xe2x80x83                        ⁢                          (                              k                ⁢                                  xe2x80x83                                ⁢                                  f                  c                                ⁢                t                            )                        ⁢                          w              ⁡                              (                t                )                                      ⁢                          rot              ⁡                              (                                  v                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                                                        (        5        )            
With this signal, when only a cycle prefix portion is regarded, the cycle prefix portion is represented by the following equation.
z(t)=(c(t)+c(txe2x88x92Tc))rot(vt)=c(t)rot(vt)+c(txe2x88x92Tc)rot(v(txe2x88x92Tc))rot(vTc)xe2x80x83xe2x80x83(6)
In this case, if d(t)=c(t)rot(vt) and a=vTc, the cycle prefix portion can be described as represented by the following equation.
z(t)=d(t)+d(txe2x88x92Tc)rot(a)xe2x80x83xe2x80x83(7)
More specifically, signals d(t) having equal waveforms are repeated such that the signals are separated from each other by 240 xcexcs and rotated by a phase a [rotation]. In a conventional frequency offset detection circuit for the OFDM wave, a frequency error was detected by using the cycle prefix component. That is, when the frequency offset detection circuit described above and shown in FIG. 3 detects the frequency offset of the OFDM wave multiplied by windowing data shown in FIG. 5, 240 xcexcs is set as an amount of delay of the delay circuit 6, and 40 xcexcs is set as a period in which averaging is performed by the averaging circuit 8. Correlation between the cycle prefix component at the start portion and the cycle prefix component at the end portion is detected by this circuit to detect a frequency error. The following [Equation 8] represents a state wherein a frequency error is detected by the circuit in FIG. 3, and a frequency error can be detected by calculating the phase of the value of a complex number q.                                                         q              =                              xe2x80x83                            ⁢                                                ∫                                      t                    =                                          240                      ⁢                                              xe2x80x83                                            [                      µs                      ]                                                                            t                    =                                          280                      ⁢                                              xe2x80x83                                            [                      µs                      ]                                                                      ⁢                                                                            d                      *                                        ⁡                                          (                                              t                        -                                                  240                          ⁢                                                      xe2x80x83                                                    [                          µs                          ]                                                                    )                                                        ⁢                                      d                    ⁡                                          (                                              t                        -                                                  240                          ⁢                                                      xe2x80x83                                                    [                          µs                          ]                                                                    )                                                        ⁢                                      xe2x80x83                                    ⁢                                      rot                    ⁡                                          (                      a                      )                                                        ⁢                                      ⅆ                    t                                                                                                                          =                              xe2x80x83                            ⁢                                                rot                  ⁡                                      (                    a                    )                                                  ⁢                                                      ∫                                          t                      =                                              240                        ⁢                                                  xe2x80x83                                                [                        µs                        ]                                                                                    t                      =                                              240                        ⁢                                                  xe2x80x83                                                [                        µs                        ]                                                                              ⁢                                                                                    "LeftBracketingBar"                                                  d                          ⁡                                                      (                                                          t                              -                                                              240                                ⁢                                                                  xe2x80x83                                                                [                                µs                                ]                                                                                      )                                                                          "RightBracketingBar"                                            2                                        ⁢                                          ⅆ                      t                                                                                                                                              =                              xe2x80x83                            ⁢                                                rot                  ⁡                                      (                    a                    )                                                  *                                  (                  real_value                  )                                                                                        (        8        )            
By the way, as shown in FIG. 1, when multi-connection is performed by an OFDM wave, as shown in FIG. 2, the frequency offset detection circuits are required for respective receiving system of signals from the mobile stations, and a base station must have a plurality of the frequency offset detection circuits each shown in FIG. 14. The base station is disadvantageously complex in arrangement.
It is an object of the present invention to make it possible to detect a frequency offset in reception state of an OFDM modulation signal with a simple arrangement.
In order to solve this problem, a receiving method according to the present invention extracts a cycle prefix component of a received OFDM modulation signal, Fourier transforms the extracted cycle prefix component, calculates an absolute value square of the component, detects periodicity of an envelope of the absolute value square value, and determines a frequency error of the OFDM modulation signal on the basis of the phase thereof.
According to the receiving method of the present invention, the phase value of the value of the periodicity of the envelope of the detected absolute value square value becomes a value which is in proportion to a frequency error of the OFDM modulation signal, and the frequency error can be determined.
A receiving apparatus according to the present invention comprises a cycle prefix component extraction means for extracting a cycle prefix component of a received OFDM modulation signal, a Fourier transformation means for Fourier transforming the extracted cycle prefix component by the extraction means, an absolute value square means for calculating an absolute value square of the component transformed by the Fourier transformation means, periodicity detection means for detecting periodicity of an envelope from an output from the absolute value square means, and a phase detection means for detecting a phase of an output from the periodicity detection means.
According to the receiving apparatus of the present invention, the phase value detected by the phase detection means becomes a value which is in proportion to a frequency error of the OFDM modulation, and the frequency error can be determined on the basis of the phase value.